Lifting magnets have been in common use for the past several decades and have become the accepted manner of handling all types of magnetic materials. The lifting capacity of any electromagnet is directly related to the ampere-turns of its coil. It makes no difference, magnetically, if there is one or a multiplicity of turns in the coil. As long as the number of turns multiplied by the number of amperes (amps) equals a particular product, one achieves the same magnetic force. For practical considerations magnets have been made with many turns, generally several thousand, and relatively low amperage, usually less than 100 amps. This choice of multiplicants results in an ampere-turn product sufficient for conventional use and permits the use of a flexible electrical conductor of convenient size to supply the necessary power.
The necessity of cooling an electrical coil used in energizing extremely powerful electromagnets of the type utilized in charged particle accelerators, has long been recognized. An example of such an electrical coil is described in U.S. Pat. No. 3,056,071 to Baker et al. Current requirements in such magnets are extremely high, being several orders of magnitude greater than that used in a scrap yard. Moreover accelerator electromagnets are not portable, at least in the sense that they may be suspended from the end of a cable on a crane, and they are unsuited to absorb the punishment to which a lifting magnet in a scrap yard is subjected.
The present invention is directed to electromagnets, such as are used in scrap yards, to lift quantities of relatively small steel pieces from a scrap pile and drop the pieces into a hopper car, comminutor, or the like. It is specifically designed to provide superior penetration into a pile of scrap, that is, pick up a deeper load than conventional lifting magnets. Superior penetration is possible because of the vacant space within the dome-shaped metal case in which scrap pieces can be held. Recognizing that much energy is wasted when there is air-space in a load, it is desirable to pack scrap more densely into the magnet case not only to increase the load picked up on each cycle, but also to increase the electrical and magnetic efficiency with which the load is picked up.
Conventional lifting magnets have a metal case within which the electrical winding is disposed. A typical metal case, as illustrated in U.S. Pat. No. 3,693,126 is cast and machined and includes flanges, usable for attaching the magnet to a lifting means, which flanges are an integral part of the metal case. The metal case also includes a central core and a bottom plate on which the electrical winding rests. Fabricating the central core to accomodate the bottom plate and machining the metal case so that all the structural components provide a hermetical seal for the electrical winding enclosed within the metal case, is arduous and expensive, requiring extensive machining. My invention provides a dome-shaped metal case fabricated from plural nested arcuate steel shells superposed one upon another in magnetic communication with a central core to form a unitary outer pole shoe which requires essentially no machining.
Typically, a scrap lifting electromagnet is operated by placing the electromagnet on a pile of scrap then energizing the coil, lifting the scrap held to the magnet, transporting the scrap to a desired location, and turning off the current to the coil so as to release the scrap. A conventional lifting magnet with the coil sealed in the metal case by a coil support plate, rests on top of a pile of scrap and the only penetration for load pickup is that generated by the magnetic field of the coil. The hollow metal case of this invention permits physical penetration of the central core and coil into a pile of scrap, before the coil is energized, thus packing scrap into the hollow case. When the coil is energized, additional scrap is attracted to and around the coil, further packing scrap into the hollow case and increasing the density of packed scrap material to increase lifting efficiency. Lifting efficiency is of less significance for lifting stacked steel plates and tightly coiled strip steel in a steel mill because, unlike for scrap of random shape and size, the air-space is relatively small. Of greater consequence in a steel mill is providing the coil of a conventional magnet with thermal protection against heat dissipated by hot steel lifted by the magnet.
U.S. Pat. No. 3,693,126 is particularly directed to handling hot steel plates which might attain a temperature as high as 1100.degree. F. As pointed out therein, one critical factor limiting the ability of an electromagnet to operate while lifting magnetic materials at such a high temperature, is the extent to which the electrical insulation of the magnet coil can withstand damage or deterioration due to the heat. To solve this problem, the reference teaches (a) a cooling medium in a coil encasing the winding, (b) circulating a cooling medium flowed over a conventionally wound solid wire electrical conductor to permit more efficient cooling of the winding due to direct contact of the cooling medium with the surface of the winding, and (c) a cooling medium in a cooling coil disposed within the electrical winding. This solution to the problem avoids damage to the insulation of the conductor due to the hot magnetic material being lifted, but it does nothing to increase the efficacy of the magnet, or to damp the buildup of eddy currents when high amperage is used. Water or coolant is not flowed through the bore of a hollow electrical conductor. The purpose of the coolant is solely to provide a thermal barrier for the insulation of a conventionally wound coil for a lifting magnet.
The importance of the effect of heat in the design of an electromagnet has been discussed in numerous publications, for example, in Knowlton's Standard Handbook for Electrical Engineers 8th Edition, Section 5, pages 182-190, but no practical solution has been provided for dissipation of the heat and simultaneous damping of the eddy currents generated due to high amperage.
A common means for cooling large high current coils uses the forced coolant technique. In this method a coil is wound of hollow conductor and a coolant is circulated through the axial passage of the conductor. One problem lies in efficiently forcing a coolant through the extremely long, restricted and curved coil passage. As stated in U.S. Pat. No. 3,056,071, if the hollow conductor is constructed with a large diameter cross-section, in order to reduce the pumping pressure required, the coil is resultingly less compactly wound, that is, has fewer turns per cross-sectional area, with deleterious results from the electrical standpoint. The overwhelming importance of having a hollow conductor through which coolant may be pumped under practical conditions, dictates that these deleterious results must be avoided. Accordingly, U.S. Pat. No. 3,056,071 teaches replacing the conventional hollow, tubular conductor with a flat strip of conductor wound in a tight spiral. One surface of the conductor is scored with parallel transverse grooves which constitute, when the conductor is wound into spiral form, a plurality of short longitudinal coolant passages distributed uniformly throughout the coil. To provide insulation between adjacent turns of the coil, a matching flat sheet of suitable dielectric material is wound with the conductor. Coolant liquid is easily pumped through the short parallel coolant channels of the coil, and in passing therethrough exteriorly of the conductor, effects an excellent heat transfer.
U.S. Pat. No. 3,693,126 to Rybak recognized that short longitudinal coolant flow described in U.S. Pat. No. 3,056,071 permits pumping a large volume of coolant in a single stream through the coil, but also recognized that this structure was unsuited to the continual impact to which a lifting magnet is subjected. Rybak therefore surrounded an electrical winding of solid conductor with a fluid-cooled jacket, and in one embodiment placed tubular cooling coils immediately adjacent and in heat-conducting relationship with the electrical winding. In so doing he effected no saving in the mass of electrical winding conventionally used for a preselected purpose, but added to the weight of the lifting magnet. This was consisent with solution of the particular problem of keeping a conventional lifting magnet cool, rather than the problem of saving weight in the magnet, and effecting a substitution of scrap payload for the weight savings. By substituting an internally cooled winding for a conventional winding a weight saving is effected which is of comparable importance to the weight saving effected by substituting a fabricated metal case for a cast case.
This is better understood by noting that consideration of weight recognizes that a crane has a specified lifting capacity which is the combined sum of magnet weight and scrap payload, irrespective of the distribution of each component. The desirability of maximizing scrap payload and minimizing magnet weight to increase lifting efficiency, is one of the problems to which this invention is directed.
In the past, both the weight of the metal case and that of the coil of an electromagnet were assumed to be immutable factors in the construction of lifting magnets. To be sure, various shapes of metal cases have been fabricated for particular purposes, as for example in U.S. Pat. No. 3,283,278, but the concept of substituting a metal case fabricated from plural arcuate nested shells, for a monolithic casting, eluded the prior art. Notwithstanding the lack of inventive faculty normally ascribed to making a substitution of any kind, for whatever purpose, it is a fact that it is not apparent that a fabricated metal case permits precisely tailoring the case for a particular magnetic field, thus avoiding the use of unnecessary material; and, suprisingly, the ease of forming rolled laminar steel sheets, and welding them along the periphery in nested relationship, contributes unexpected economies over casting and machining a housing, both in manufacture and in repair and maintenance. Whatever the reasons that the substitution was not disclosed in the prior art, it is now established that the fabricated metal case (a) is from about 20 percent to about 80 percent less in weight than a cast case for an electrical coil of preselected performance; (b) avoids the difficulties, risks and capital expenditures which attend the production of a large casting; and, (c) permits dents and breaks resulting from the rough treatment to which scrap yard lifting magnets are subjected, to be easily repaired.
Though weight of a lifting magnet is a key factor, lifting efficiency also depends upon the cycle time required to pick up, lift and drop off a load of magnetic material. Thus, for example where a lifting magnet is used as in a scrap yard, to pick up and release scrap metal, this cycle being repeated continuously throughout the working day, both the amount of scrap which may be picked up by the magnet and the rate at which it may be transferred are limited by the design of the electromagnet. As of the present time, to the best of my knowledge, no one has utilized a slotted case and central core structure in which eddy current effects are damped, nor has the concept of utilizing plural arcuate shells, nested one with another to provide a magnet case, been utilized to house a hollow electrically continuous conduit in which the cooling fluid path is discontinuous. By the term discontinuous is meant that plural fluid paths are provided for cooling the coil, any one of which may be blocked without interfering with the fluid flow through the others.
Finally, electrical coils used for energizing extremely powerful electromagnets of the type utilized in charged particle accelerators, particularly where such magnets are of the pulsed variety, do not have an iron case or central iron core because the magnetic fields are generally in excess of that required to saturate iron. Since there is no iron case or core the problem of eddy current buildup is not a serious consideration even if the field is turned on and off numerous times. In a large lifting magnet for scrap, however, eddy current buildup is so significant that it may be 15 seconds, after the current is turned on, before the magnet can pick up its load; and another 15 seconds, after the current is turned off, before the magnet can drop the load.